Simplify the following expression: $9\sqrt{99}$.
Solution: First, try to factor a perfect square out of the radical. $= 9\sqrt{99}$ $= 9\sqrt{9 \cdot 11}$ Separate the radical and simplify. $= 9\sqrt{9} \cdot \sqrt{11}$ $= 9 \cdot 3 \cdot \sqrt{11}$ $= 27\sqrt{11}$